This invention relations to a method and apparatus for use in the testing of an electrical component. More especially, the invention relates to a means of testing an electrical characteristic at a given frequency in such a manner as to avoid the effects of interference occurring in a test circuit at the said given frequency. The invention is particularly applicable to testing to be effected in a power system in which interference may occur owing to spurious voltages and currents induced in the test circuit owing to the presence of power line frequency fields.
More especially, the invention relates to a method and the implementation of that method to be used as a means of measuring the leakage impedance and loss angle of a high voltage component or insulation system as a tool by which the condition of said component or insulation system can be assessed as part of the installation and maintenance procedures.
The condition of insulation systems can be assessed by measuring the loss angle (or xcex4), which gives a measure of the performance of the insulation. An ideal insulation system behaves as an ideal capacitor in that when the system is energised with an alternating voltage, the current that flows in the insulation system is exactly 90xc2x0 out of phase with the voltage. A real insulator, though, has a finite resistance that appears in parallel with the ideal capacitance, which causes an energy loss when the system is energised. This resistance reduces the phase angle of the current with respect to the voltage. The angle of this phase shift is the loss angle (xcex4). The higher the loss angle, the more resistive the insulation system is. The loss is normally measured in terms of tan (xcex4), which is given by C/R where C is the capacitance of the insulation system and R is the loss resistance. One of the indicators of a degrading insulation system is the value (xcex4) increasing over time.
Tan (xcex4) has been measured in the past using bridge balancing methods such as the Schering bridge, or an inductively coupled ratio arm bridge (e.g. Tettex type 2805). Later systems have used direct measurement of the voltage and current with electronic processing of the resulting signals to measure the tan (xcex4) factor. One of the limitations with this method is the presence of power line frequency fields that induce spurious voltages and currents onto the test object and the system. These prevent a true measurement of the loss angle being made.
Ways of reducing the effect of this interference have been developed. The simplest is to choose a frequency that is slightly away from the line frequency, using this to energise the test object. The voltage and current are measured using synchronous detection or interference of the waveforms. This method relies on the assumption that the loss angle is constant with respect to frequency, which cannot be guaranteed an alternative is to measure on either side of the line frequency and perform a linear interpolation of the results to establish the loss angle at the line frequency. Hitherto this has required the use of complex synchronous schemes requiring accurate phase shift elements or multiple discrete measurements with complex digital signal processing noise suppression.
It is accordingly an object of the present invention to provide an improved method and means that overcomes, or at least reduces, the disadvantages referred to above.
In accordance with one aspect of the invention there is provided a method of measuring the insulation impedance of a capacitative electrical component at a power line frequency comprising the steps of generating a test signal having a composite waveform consisting of two sinusoids at first and second frequencies respectively above and below said power line frequency, applying said test signal to a test circuit including said electrical component, sampling the applied test signal, synchronously with the generation thereof, at a sampling frequency that is an integer multiple of each of said first and second frequencies, for a sampling period corresponding to an integral number of cycles of each of said first and second frequencies, in order to derive and store corresponding digital values of said applied test signal, calculating, from said stored values, for each of said frequencies, values of voltage applied to and current flowing in said electrical component in order to derive respective values corresponding to the insulation impedance of the component at said frequencies, and interpolating from said derived values to obtain a value at the power line frequency.
The invention also provides an apparatus for use in measuring a characteristic of an electrical component or circuit at a given frequency, comprising means for generating a test signal having a waveform consisting of two sinusoids at different frequencies respectively above and below said given frequency, means for applying said test signal to a test circuit, means for sampling values of signals at points in said test circuit synchronously with the generation of said test signal, at a sampling frequency that is an integral multiple of each of said different frequencies, for a sampling period corresponding to an integral number of cycles of each of said different frequencies, and for storing said sampled values in digital form, and data processing means programmed to process said stored values in order to derive, by interpolation from values calculated for said different frequencies, the value of said characteristic at said given frequency.